I realize this thread is old, but I wanted to share my experiences. I'm completely new to Android but I've done a lot of research and did so before purchasing my GS5.
I've only had the phone a couple days but I've been messing with all sorts of apps and tweaks and have had my phone on ART since this morning. At times it seems just a tad buggy, but overall it runs smooth. I tried it with animation scales off but it actually performs smoother with them scaled up to .5 again. Everything from screen rotation to app launching/notification center actions all seem smoother. Battery life seems better too, but I haven't done a ton of use to be super confident about this aspect yet.